Presently, an optical fiber for use in commonly popular optical fiber communication systems has a structure in which the outer circumferential surface of one core is surrounded by a cladding. Optical signals propagate through the inside of the core to transmit information. In these years, with a widespread use of optical fiber communication systems, the volume of information to be transmitted is dramatically increased. With an increase in the volume of information to be transmitted, in an optical fiber communication system, a large number of optical fibers ranging from a few tens to a few hundreds optical fibers are used to conduct large-capacity long distance optical communications.
In such an optical fiber communication system, a method is known in which with the use of a multicore fiber having one cladding surrounding the outer circumferential surfaces of a plurality of cores, light beams propagating through the cores transmit a plurality of signals.
Patent Literature 1 below describes an example of a multicore fiber. In this multicore fiber, one core is disposed in the center of a cladding, and six cores are disposed around the core disposed in the center. This disposition is a structure in which cores can be closely packed. Thus, many cores can be disposed to the outer diameter of a specific cladding. In the multicore fiber described in Patent Literature 1, in order to reduce the crosstalk between light beams propagating through the cores, the effective refractive indexes of the cores adjacent to each other are set different from each other.
However, there is a demand to further reduce crosstalk than in the multicore fiber described in Patent Literature 1 in which the effective refractive indexes of the cores adjacent to each other are changed. Therefore, a multicore fiber is known in which a low refractive index layer having a refractive index lower than the refractive index of a cladding is disposed to surround the outer circumferential surfaces of cores, further preventing crosstalk. When this multicore fiber is viewed from the viewpoint of the refractive index, the low refractive index layer has a trench shape. Thus, the multicore fiber is referred to as a trench-assisted multicore fiber, and the configuration from the core to the low refractive index layer is referred to as a core element.
However, in this multicore fiber, when the core elements are disposed to surround a certain core or core element, it is difficult to release higher mode light beams in a light beam propagating through this certain core or core element, causing a cutoff wavelength to become a long wavelength. Therefore, like a multicore fiber described in Patent Literature 2 below, a configuration is known in which in a plurality of core elements surrounding a certain core or core element, a part of the core elements is formed in a simple core by removing a trench layer from the core element. According to such a multicore fiber, a cutoff wavelength can be prevented from being a long wavelength, while reducing crosstalk.    [Patent Literature 1] JP2011-170336 A    [Patent Literature 2] International Publication No. WO 12/118132